I think this gives the correct answer, but the computation is different (which is fine - we're only looking at the answer); K over/scan go left-to-right, and if I'm not mistaken, APL and J go right to left, that is, in K:

    +/!5 equals ((((1+2)+3)+4)+5) which is not 1+2+3+4+5

whereas in APL/J it is

    +/iota5 equals (1+(2+(3+(4+5)))) which is 1+2+3+4+5

I find the APL approach mathematically consistent and elegant, but the K approach is more practical - it makes over and scan useful for state machines, parsing, and many other uses without having to reverse anything twice.

But often, as in this case (or the flatten case) it doesn't matter which direction you start folding from.

K does a thing (like J I think?) where it's scan/reduce returns each subsequence/intermediate-value as it goes which is what makes that problem so trivial in K.